The gantry mounting radiography apparatus is a sophisticated X-ray electronic apparatus that may be used, in particular, to perform arteriographies. Arteriographies are the examination of the arteries using X-rays following an injection of a radiopaque substance. Since the X-ray images are in two dimensions, X-rays are taken at different angles to be able to get a three-dimensional view of arteries and their possible pathologies, such as aneurysms. A digital subtraction technique is used to isolate the injected radiopaque substance from the global X-ray image. According to this technique, a first X-ray image is taken before the injection of the radiopaque substance and a second is taken immediately after the injection. The two images are digitalized and subtracted in a computer The radiopaque substance is then clearly visible on the resulting image to show the details of the analysed arteries. In a gantry mounting apparatus, images are taken at different angles in a rapid succession so that only one injection of radiopaque substance is necessary.
When making an arteriography, the hyper radiation due to air inside or outside the analysed organs seriously degrades the images by saturating some areas. No useful information is obtained from a saturated area because the halation creates an overexposed zone. Yet, compensating the hyper transparency of an organ, such as the lungs inside the human body, is quite delicate. On one hand, the hyper radiation is very difficult to compensate electronically due to the inherent remanence of the X-ray tube amplifier of the apparatus. On the other hand, a common solution is to restrict the size of the area to analyse by closing the diaphragm of the X-ray source in order to avoid the halation due to air at the periphery of the body, such as the periphery of the head or the neck of the patient. However, restricting the analysed area may prevent the specialist from seeing the peripheral arterioles and thus some possible pathologies.
Some attempts were made for correcting the saturation problems by using metal compensating plates, curved or wedged, set directly against the patient or near the image device. Although sufficient for a simple radiography, they are not always adequate for all incidences required in multiple angle arteriography. For example, U.S. Pat. No. 4,472,828 describes an X-ray filter for chest X-rays which operation principle is similar to the one of the metal compensating plates This device may improve the image of the radiography about the thorax, but it is stationary and as a result, it limits radiography to one view of the organ at a time and has to be reset for a different view.